The Helix resonator is generally used in filters operating in the frequency range of 100-1000 MHz. The resonator comprises an inner conductor, which is wound into a spiral coil, and the outer one is ,a metallic box that surrounds the former. One end of the coil can be connected directly to the box and in practice this is usually done by making the conductor, which is wound into the spiral coil, at this end straight for some distance and fixing it approximately perpendicular to-the end surface of the resonator box. The first round of the spiral coil is therefore situated at a distance from the end surface of the box, as defined by this straight leg. The opposite, open end of the coil is separate from the box and is capacitively coupled to the box. Electrically the resonator forms an LC-resonator circuit. The resonator can be connected electrically to the rest of a filter circuit by not connecting one end electrically to the box, but by instead connecting it with a connection lead which has been isolated from the box, or by attaching to a certain part of the Helix resonator coil a connection lead which goes insulated through the box wall. Mechanically the resonator coil can be of the vertical type i.e. the resonator coil is surrounded around the same axle by a metallic box which is earthed. The resonator coil is sometimes fastened on a support plate before it is inserted in the box. The position of the coil in relation to the support plate can be upright or lying down.
By connecting several resonators to cascade, it is possible to construct a filter with favourable characteristics, e.g. a duplex filter. The filter has to be designed so that the stop and passband characteristics do not change e.g. due to vibration occuring in mobile telephones. Because of this the Helix resonators of duplex filters must be supported mechanically in such a way that they cannot move.
One known way is to wind the loops of the coil around a cylindrical, hollowlike body of insulation material, which in turn is supported in various ways on the box construction. The disadvantage of this solution is that the body material in the electric and magnetic field of the coil lowers the Q value of the resonator.
Another known way to support the coil is that, after the coil has been wound, a plastic U-shaped binder ring is pressed around the loops of the coil. The loops of the coil run through the spaces in the binder ring and the part that connecting the arms is fastened to the installation plate. This way of supporting also lowers the Q value of the resonator, because the U-shaped ring used for supporting is in the middle of the electric and magnetic field of the resonator. The Q value is considerably lower compared to a resonator where no supporting means has been used in the electric and magnetic field. Another disadvantage is that the mould pressing of the plastic is a relatively complicated work procedure, where the variation in the amount of plastic in the binding is hard to control and may lead to rejects.
In the Finnish patent application number 884503, a Helix resonator is presented, in which a protruding part, preferably a curve, is formed on one of the resonator loops, this part resting against a small metallic folio strip on the circuit board. On the opposite side of the circuit board is another metallic folio strip, which is earthed. The strips and the material of the circuit board form a condensator, which acts as a simple temperature compensation for the resonator. The presented construction does support the resonator, but its disadvantage is the losses caused by the so called "excess" circuit board material, which occurs because the electrical field is in a lossy circuit board material at the supporting point.